For meeting the requirements of such vehicles, many combinations have already been proposed.
The most elementary is made of a motion reversal device comprised of a planetary gear reverser followed by 3 simple planetary trains and a direct drive: thus are obtained 4 forward speeds and 4 reverse speeds. But since the reversing gear made of planetary gears comprises itself two planetary trains, this leads to an assembly comprising 5 planetary trains resulting in a highly complex mechanical system which does not allow a compact assembly.
According to another combination, a reversing planetary gear is used, followed by two simple planetary trains supplying each twice 2 combinations.
This solution is more modern but, as the previous one, is limited to 4 cominbations for the forward motion, which is insufficient in many cases, particularly in the case of trucks and other modern transportation vehicles.